The invention relates to a hot-gas engine, having at least one closed working space in which a working medium performs a thermodynamic cycle during operation, and arranged so that heat originating from a heat source is supplied to the said working medium via a heater having one or more ducts through which working medium flows during operation, and more particularly to a hot-gas engine which also includes one or more reservoirs containing a meltable material which stores heat originating from the heat source.
A hot-gas engine of this kind is known from Swiss patent specification No. 512,670, to which U.S. Pat. No. 3,791,136 corresponds.
In the known hot-gas engine, the flue gases originating from a burner device give off part of their heat directly to the working medium by flowing past heater pipes or ducts through which working medium flows, and part of their heat directly to the meltable heat-storing material in the reservoir where the flue gases flow along wall portions of this reservoir.
However, direct heating of the meltable heat-storing material in the reservoir by the flue gases can introduce some problems.
Because the meltable material, usually a salt such as LiF, CaF.sub.2, SrF.sub.2 or a mixture of salts, has a low heat conductivity in the molten as well as in the solid state, the reservoir walls along which the hot flue gases flow assume a very high temperature. This causes fast corrosion of these reservoir walls, on the flue gas side as well as on the side of the storage material, which normally contains impurities having a corrosive effect.
The risk of burning through and/or cracking of the reservoir walls is high, particularly when such salts are in the solidified state. This is because the usable salts shrink substantially upon solidification (volume reduction in the order of magnitude of 30%), so that the contact between the salt and the reservoir walls is substantially broken and the wall portions heated by the flue gases are not cooled by direct conduction.
The choice of thick reservoir walls is not attractive in view of the weight and the dimensions of the engine and, moreover, it does not offer a satisfactory solution to the problem of fast corrosion. Maintaining a maximum flue gas temperature which is substantially higher than the melting temperature of the salt but lower than the temperature limit imposed for the material of the reservoir walls constitutes a difficult control problem for the hot-gas engine with its variable load. The temperature fluctuations in the flue gases would then have to be limited to plus or minus 50.degree. C.